adding Promise::await

adding Promise::await

Summary:
This diff adds a static await method to Promise.
Also after this, folly::fibers::await is just a wrapper around Promise::await.
This also removes the friend relationship between Promise and await.

This would be helpful in making the folly::fibers::await templated by Baton changes easier to make.

Reviewed By: andriigrynenko

Differential Revision: D3314891

fbshipit-source-id: 361546c078caafd067734d2f474c617d7fb888b0